Gas chromatography (GC) is used to separate solutes or components in an analyte sample (vapor) for measurement. A gas chromatography mass spectrometer (GC-MS) is an implementation in which the GC device provides molecular samples in gaseous form to an inlet of the MS device. Generally, a GC device includes capillary column(s) for separating the solutes. The columns are typically made of metal, glass or quartz, for example, and coated on the inside with a thin-film coating or stationary phase. The GC process consists of introducing the analyte sample into a column using a continuous flow of carrier gas, such as hydrogen or helium. Various solutes within the sample react differently with the stationary phase, and thus move at different speeds through the column, resulting in separation of the solutes. The separated solutes may then be detected by various detectors or provided as input to a mass spectrometer.
Two-dimensional GC (2D-GC) systems include two columns, arranged in series, having different dimensions and/or different stationary phases. This allows for additional separation of the solutes in the sample under different conditions, which is particularly effective for solutes having similar reactions to the first stationary phase. Two-dimensional GC systems include modulators, which trap and accumulate solutes from the first capillary column, while compounds from the second capillary column are being analyzed. Two-dimensional GC systems may have either flow based or thermal based modulators. Typically, conventional flow based 2D-GC modulators are relatively small in size and do not need cryogens or other cooling means for operation. However, flow based 2D-GC modulators use about ten times more carrier gas, all of which flows out of the second dimension column. Therefore, in 2D-GC-MS applications, for example, column flow must be split prior to the mass spectrometer, allowing only about 1/10 of the carrier gas into the mass spectrometer, and thus causing a reduction of detection sensitivity.
Conventional thermal based 2D-GC modulators typically introduce at least one non-moving cryogenic gas jet into a hot GC oven, usually impinging at the starting segment of second dimension capillary column to affect solute trapping. Examples of conventional thermal based 2D-GC modulators include dual jet design, quad-jet design including two cold jets and two hot jets, and loop modulator design including one cold jet and one hot jet. Conventional cryogenic thermal modulators are also provided with moving mechanisms, such as a moving cryogenic modulator, a semi-rotating cryogenic modulator, and a longitudinally modulated cryogenic system (LMCS), which is described for example, by MARRIOTT in Australian Patent No. AU 199748570, “Apparatus and/or Device for Concentration.”
However, even though only a small segment of a capillary column is to be cryogenically cooled in thermal modulator designs, the conventional thermal based 2D-GC GC modulators perform cooling inside the hot GC oven, so that the ambient heat of the GC oven may be used to effect solute remobilization (e.g., before and after cooling). Accordingly, large amounts of gas jet at a very low temperatures must be produced, most of which is needed simply to nullify the heat of the surrounding hot GC oven. Production of cold gas jets may be either from insulated tanks storing large amounts of cryogenic fluids (e.g., liquid nitrogen, liquid carbon dioxide and the like), or with a refrigeration system having substantial cooling power. The conventional 2D-GC modulators are therefore bulky in overall size, and consume extensive amounts of power and cryogens.